1. Field of the Invention
The present invention relates to a down-traffic-signal transmission system for a mobile communication system in which communication between base stations and mobile stations is performed in accordance with the CDMA (Code Division Multiple Access) scheme.
2. Description of the Related Art
In a mobile communication system in which communication between base stations and mobile stations is performed in accordance with the CDMA scheme, when routing is switched from one base station to another base station in accordance with movement of a mobile station, such switching must be performed instantaneously such that a user does not notice it. Therefore, during switchover of base stations, a mobile station communicates with two or more base stations simultaneously (this technique will be called xe2x80x9csoft handoffxe2x80x9d hereinafter). At this time, the mobile station performs maximum ratio composition in which energies of signals received from the respective base stations are added together.
In the CDMA scheme, in order to allow a mobile station to communicate with a plurality of base stations simultaneously, each of the base stations modulates a signal through use of a different spread code, and the mobile station demodulates the signal through use of the same spread code as that used in the corresponding base station. Thus, the mobile station can extract the signal transmitted from each base station.
For example, International Patent Publication No. WO 95/08901 discloses a technique in which signals are modulated at respective base stations through use of different spread codes; the signals modulated through use of different spread codes are respectively received by means of independent reception units provided in a mobile unit; and demodulated signals output from the reception units are combined.
With reference to FIG. 6, a description will be given of such a conventional mobile communication system, particularly of a system for transmitting a down traffic signal from a base station to a mobile station.
FIGS. 6(a) and 6(b) show the structure of a conventional down-traffic-signal transmission system, wherein FIG. 6(a) is a block diagram of the down-traffic-signal transmission system, and FIG. 6(b) is a block diagram showing the structure of a spread-coding apparatus provided in a base station.
In FIG. 6(a), numeral 51 denotes a base station controller 51 which establishes connection between a mobile communication exchange and base stations and controls the base stations. First and second base stations 52 and 53 are connected to the base station controller 51 via respective lines. A mobile station 54 communicates with the first and second base stations 52 and 53 via respective radio means. although FIG. 6(a) shows an example in which the mobile station 54 communicates with two base stations (the first and second base stations 52 and 53), three or more base stations may be installed.
As shown in FIG. 6(b), each of the first and second base stations 52 and 53 has a spread-coding apparatus 60 which performs modulation through use of a spread code. The spread-coding apparatus 60 includes a modulator 64. The spread-coding apparatus 60 of the first base station 52 performs modulation through use of a spread code C1, and the spread-coding apparatus 60 of the second base station 53 performs modulation through use of a spread code C2.
In the first and second base stations 52 and 53 there are composed a voice signal and a control signal (e.g., a signal for identifying a base station under communication)xe2x80x94which are separately transmitted from the base station controller 51xe2x80x94and a thus-obtained composite signal is input to the spread-coding apparatus 60 as a traffic signal (input signal). An output signal from the spread-coding apparatus 60 is transmitted to the mobile station 54 as a down traffic signal.
The down traffic signal is not necessarily composed of both a voice signal and a control signal and in some cases may be composed of only a voice signal or a control signal. Further, the mobile station 54 includes reception units that can demodulate signals modulated through use of the spread codes C1 and C2.
However, in the above-described conventional down-traffic-signal transmission system, when a fault occurs in communication between the base station controller and the base stations, the base stations may send different down traffic signals to the mobile station.
For example, if the first base station correctly receives a voice signal and a control signal and the second base station correctly receives only the voice signal at a timing where both the voice signal and the control signal have to be received by the first and second base stations, the mobile station receives different signals from the first and second base stations. In such a case, the mobile station cannot compose them, and therefore cannot reproduce a signal.
In order to solve such a problem, the down-traffic-signal transmission system may be modified such that a base station that has correctly received only a voice signal or a control signal does not transmit a down traffic signal to the mobile station. However, in this case, the voice signal or the control signal that is correctly received is not used in the mobile station. Further, if the first base station correctly receives only the control signal and the second base station correctly receives only the voice signal, no down traffic signal is transmitted to the mobile station.
The present invention has been accomplished to solve the above-described problems involved in the prior art techniques, and an object of the present invention is to provide a down-traffic-signal transmission system that allows a mobile station to effectively use a correctly received signal even when a base station correctly receives only one of a voice signal and a control signal transmitted from a base station controller.
Another object of the present invention is to provide a down-traffic-signal transmission system that can prevent a signal that is not correctly received by the base station from being received by a mobile station as noise that has a strong correlation with a correctly received signal and therefore makes reproduction of the signal impossible.
To achieve the foregoing objects, the present invention provides an improved down-traffic-signal transmission system for a mobile communication system comprising:
a plurality of base stations;
a mobile station capable of being connected with at least one base station via radio means; and
a base station controller provided for connecting a mobile communication exchange and the base stations and for controlling the base stations;
wherein communication between the base stations and the mobile station is performed in accordance with the CDMA scheme.
The down-traffic-signal transmission system is designed such that each of the base stations has a spread-coding apparatus for modulating, through use of a different spread code, a traffic signal obtained through composition of a voice signal and a control signal separately received from the base station controller; and the thus-modulated traffic signal is transmitted from the base station to the mobile station.
The spread-coding apparatus comprises:
a valid signal determination unit for determining, as valid information, a control signal and/or a voice signal contained in a traffic signal from the base station controller when correctly received, and determining, as BE invalid information, the control signal and/or the voice signal when incorrectly received;
a selection instruction unit for outputting a timing signal based on results of the determination of the valid signal determination unit, the timing signal indicating that the control signal is valid or invalid information and that the voice signal is valid or invalid information;
a selection unit which, in accordance with the timing signal, assigns to the control signal and/or the voice signal determined as valid information a spread code that can be demodulated in the mobile station, and assigns to the control signal and/or the voice signal determined as invalid information a spread code that cannot be demodulated in the mobile station; and
a modulator for modulating the control signal and the voice signal through use of the spread code assigned by means of the selection unit.
The spread-coding apparatus may have a division pattern recognition unit for recognizing the format pattern of the traffic signal formed by the control signal and the voice signal, and the selection instruction unit may operate to output a timing signal based on the result of determination of the valid signal determination unit in accordance with the format pattern that the division pattern recognition unit has recognized.
In the down-traffic-signal transmission system having the above-described structure, in accordance with the timing signal, the selection unit assigns to the control signal and/or the voice signal determined as valid information a spread code that can be demodulated in the mobile station and assigns to the control signal and/or the voice signal determined as invalid information a spread code that cannot be demodulated in the mobile station.
Accordingly, when a mobile station communicates with a plurality of base stations during, for example, soft handoff, the mobile station demodulates control signals and voice signals correctly received at the base stations among the signals transmitted from the base station controller to the base stations.
At this time, a control signal and/or a voice signal that has not been correctly received at a base station is modulated through use of a spread code that has no correlation with a spread code that can be demodulated at the mobile station. Therefore, the incorrectly received signal or signals do not serve as noise having a strong correlation with a correctly received signal.
The present invention has the following advantageous effects:
Even when a base station correctly receives only one of a voice signal and a control signal transmitted from the base station controller during soft handoff, the mobile station utilizes signals that are correctly received at the respective base stations in order to reproduce a signal through maximum ratio composition.
Further, since a signal that is not correctly received at the base station is not received by the mobile station as noise having a strong correlation with a correctly received signal, the reproduction of a signal is not prevented by the noise.
In the case where the spread-coding apparatus has a division pattern recognition unit, and the selection instruction unit operates to output a timing signal in accordance with the format pattern that the division pattern recognition unit has recognized, even when the amount of a control signal completely differs from that of a voice signal, both the signals can be effectively used at the mobile station. Especially, even when the down traffic signal is composed of only a voice signal or a control signal, these signals can be effectively used at the mobile station.